My local stores and butchers don't carry much tri-tip meat, so I've only cooked it a few times. I turn to sirloin steak and strip steak a lot. Unfortunately, I don't have many personal good tips for tri-tip, but I know a lot of people who love it.
It's my understanding that you want to sous vide tri-tip to a steak-like temperature. You could select 131°F (55°C) if you like a medium rare steak. Some people enjoy tri-tip just heated through for 2 or 3 hours but others like to cook it a little bit longer to tenderize it. Mike says tri-tip is one of his favorite cooks. He does 134°F (56.6°C) for 12 hours and it comes out great.